please define term "tainted coin" exactly!
i would love to have a way to check any deposit beforehand.
"Tainted" means we have received a request for investigation from the Japanese law enforcement (either the CyberPolice, or an international investigation backed by a court). In some cases we may cooperate with other law enforcement around the world on a case by case basis. As of today there is only one active case, and in the past 12 months, we have marked some bitcoins on only 3 occasions.
Anyway, laws in most countries means that any exchange operating legally is required to act upon knowledge of stolen properties, and prevent to the best of their ability resell of such (failure to do so causes the exchange to be considered accomplice and can cause the people running it to face various charges depending on the context). That doesn't mean that just anyone can come and see us asking to get bitcoins marked. We need some proof and law enforcement to be involved.
Because a Bitcoin exchange deals with both the Bitcoin world and the current financial (legacy?) world, we have to comply with rules from both sides. This means we are fated to be hated by both bankers around the world for supporting something that aims at replacing them, and bitcoin supporters for complying with outdated regulations. Our goal is to find the right balance between both worlds and create a kind of bridge to allow for easier adoption of bitcoin (if it was totally impossible to exchange bitcoin against fiat, we'd have some issues finding Bitcoin users).
If you have a way to determine that a certain amount of bitcoins on one of your accounts is stolen and this way of determination is likely to hold in court (which is, to my knowledge, not the case at this point, and furthermore there was no clear statement by which the owners of the victimized accounts could verify the legal relevance of your method), there is still no reason to penalize whole accounts instead of
just freezing the exact amount in question upon deposit and immediately informing the account owner so he can take legal steps against the accusation on his own. I understand that the policy you chose creates much more profit for you, but on the other hand you will understand that your policy will make you look rather untrustworthy in the bitcoin community.